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A small body, composed of ice and dust, in orbit around the Sun. The name derives from the Greek kometes, meaning ‘long-haired’. Comets are thought to exist in vast numbers in the Oort Cloud and Kuiper Belt, beyond the planets. From there they can be perturbed by the gravitational influence of passing stars into new orbits that bring them into the inner Solar System, where they become visible from Earth. When a comet is far from the Sun its nucleus is frozen solid and shines only by reflecting sunlight. As the nucleus nears the Sun it heats up and releases gas and dust, forming first a coma and, in some cases, a tail (see coma, cometary; nucleus, cometary; tail, cometary). The gas becomes ionized and emits light. Whereas the nucleus may be only 1 km or so across, the coma can extend for 105 km or more from the nucleus and the tail for 108 km}. Around the visible coma is an even larger cloud of hydrogen, detectable at ultraviolet wavelengths. Despite their size, a comet's coma and tail are of such low density that background stars can be seen through them. The mass of a typical comet is perhaps 1014 kg.

Each year over 200 comets are seen with telescopes and space satellites; only a few ever become bright enough to be visible with the naked eye. Most are new long-period comets appearing for the first time, with orbital periods of over 200 years. The remainder are periodic comets, either new discoveries or known objects following predicted orbits. The most famous of these, and the brightest, is Halley's Comet. At the end of 2010 over 3000 comets were known, of which over 90% are long-period comets. During their passage through the inner Solar System comets can have their orbits altered by the gravitational influence of the planets, notably Jupiter. One spectacular example was Comet Shoemaker–Levy 9, which hit Jupiter in 1994.

Some comets are discovered by amateur astronomers conducting deliberate searches, but most are found on images taken by professional astronomers; recently, over a hundred comets a year passing close to the Sun have been found on images taken by the Solar and Heliospheric Observatory (SOHO) (see Kreutz Sungrazer; Sunskirter). Comets are named after their discoverers (now usually restricted to two names), or the spacecraft or survey which found them, and are also assigned a designation based on when they were discovered. According to a convention introduced in 1995, comets are identified by the year and a letter indicating the half-month in which they were discovered, plus the order of discovery in that half-month (e.g. C/1999 D3 would be the third comet discovered during the second half of 1999 February). The names of periodic comets are preceded by P/ and a number indicating the order in which their periodicity was established (e.g. 1P/Halley, 2P/Encke). Comets that are defunct—either observed to have disintegrated or simply disappeared—are given the prefix D/ (e.g. 3D/Biela, D/1993 F2 Shoemaker–Levy). Comets for which there are insufficient observations to calculate an orbit are given the prefix X/.